Helix travelling wave tube assembly method and apparatus



1964 R. F. STEWART ETAL 3,160,943

HELIX TRAVELLING WAVE TUBE) ASSEMBLY METHOD AND APPARATUS Filed July 18. 1960 8 Sheets-Sheet l M T m mm mam TE qafiuaai im M m .wAl Q M m m m wu\ $21k K\.Q\R

qQ 6R ATTORNEY HELIX TRAVELLING WAVE TUBE ASSEMBLY METHOD AND APPARATUS Filed July 18, 1960 Dec. 15, 1964 R. F. STEWART ETAL 8 Sheets-Sheet 2 RAYMOND F. STEWART DANIEL E. WHEELER n Qst INVENTORS ATTORNEY R. F. STEWART ETAL 3,160,943

HELIX TRAVELLING WAVE TUBE ASSEMBLY METHOD AND APPARATUS Filed July 18, 1960 Dec. 15, 1964 I 8 Sheets-Sheet 3 RAYMOND E STEWART DANIEL. E WHEELER INVENTORS ATTORNEY Dec. 15, 1964 R. F. STEWART ETAL HELIX TRAVELLING WAVE TUBE ASSEMBLY METHOD AND APPARATUS Filed July 18, 1960 Q Q r i il'llll g 8 Sheets-Sheet 4 93D m 9 g Q RAYMOND F STEWART DANIEL E. WHEELER INVENTORS ATTORNEY Dec. 15, 1964 R. F. STEWART ETAL. 3,160,943

HELIX TRAVELLING WAVE TUBE ASSEMBLY METHOD AND APPARATUS 8 Sheets-Sheet 5 RAYMOND F. STEWART DANIEL E WHEELER INVENTORS Filed July 18, 1960 ATTORNEY Dec. 15, 1964 R. F. STEWART ETAL 3,160,943

HELIX TRAVELLING WAVE TUBE ASSEMBLY METHOD AND APPARATUS Filed July 18, 1960 8 Sheets-Sheet 6 FIG. 17

RAYMOND F. STEWART DANIEL E. WHEELER INVENTORS ATTORNEY FIG. I9

Dec. 1964 R. F. STEWART ETAL 3,160,943

HELIX TRAVELLING WAVE TUBE ASSEMBLY METHOD AND APPARATUS Filed July 18, 1960 8 Sheets-Sheet 7 DANIEL E WHEELER INVENTORS ATTORNEY R. F. STEWART ETAL Dec. 15, 1964 HELIX 8 Sheets-Sheet 8 Filed July l8. 1960 5 nu l types.

United States Patent 3,160,943 HELIX TRAVELLHIG WAVE ASSEMBLY METHGD AND AKPARATUS Raymond F. Stewart, Santa Cruz, and Daniel E. Wheeler,

Soquel, (Salli, assignors to Stewart Engineering Company, Santa Cruz, Cmii, a corporation or Qalifornia Filed July 18, 196%, Ser. No. 43,659 22 Claims. (til. 29--2.l5)

This invention relates generally to an assembly method for travelling wave tubes and to an apparatus for use therein.

As is Well known, travelling wave apparatus operates on the exchange of energy between an electron beam and an electromagnetic wave which is suitably coupled thereto. Generally, the electromagnetic wave is propagated along a periodic structure with its phase velocity in the direction of the electron beam substantially equal to the velocity of the beam. There is a continuous interaction between the wave travelling along the periodic structure and the electron beam. As a result of this interaction, energy is transferred from the beam to the electromagnetic wave.

As is well known, travelling wave tubes are of two The first type are those in which the group and phase velocity of the electromagnetic waves are in the direction of the electron beam. These tubes are referred to as forward travelling wave tubes. The second typ of travelling wave tubes are those in which the phase velocity of the electromagnetic wave is in the same direction as the electron beam but the group velocity (energy flow) is opposite thereto.

Operation of both these tubes is inherently the same. The electron stream has a velocity which is slightly greater than the phase velocity of the electromagnetic wave travelling along the periodic circuit. There is interaction between the electromagnetic wave and the electron beam, and the electrons in the beam are velocity modulated, that is, the electrons are bunched. The bunching takes place by some of the electrons speeding up, while others are slowed down. If the speeding up and slowing down is exactly the same, no amplification of the wave results. Fortunately, it turns out that the two quantities are not exactly equal. There is a bunching around the slow moving electrons thus reducing the mean velocity and hence the kinetic energy of the electron beam as a whole. This increment of kinetic energy is transferred to the electromagnetic wave and results in the wave being amplified.

Both of the foregoing types of travelling wave tubes may include a periodic structure of thehelix type. Generally, the helix is wound with Wire, ribbon or tape. The

electromagnetic energy travels along the wire at the speed of light. However, it travels in an axial dircction'at a velocity which is dependent upon the pitch of the helix. The pitch is selected whereby the axial velocity of the electromagnetic wave corresponds generally to the velocity of the electron stream associated therewith.

The pitch of the helix, particularly in a backwar ture and one portion of the structure may oppose the remainder of the structure. For example, in a backward wave oscillator, this may mean that it will not operate the same at all frequencies. .It may have voids in its output at certain frequencies, or it may operate at several frequencies at one time.

' the helix to firmly support the same.

frequency modulated. In many applications, frequency variations cannot be tolerated. For example, the output of a backward wave oscillator which operates over a 2:1 frequency band in the range of 5 kmc. and higher may be modulated due to changes in pitch during operation as much as several thousand cycles.

Many attempts have been madein the prior art to properly support a helical transmission line within a vitreous envelope. For example, in certain applications, the glass has been shrunk onto the helix in such a manner that ridegs are formed which embed spaced points of In other applications, a plurality of spaced rods are inserted between the tube envelope, which can be metal, and helical conductor. Spring metal clips are employed for providing the desired seating pressure between the parts. Another attempt at firmly holding a helix within an envelope has been to partially embed the turns in a vitreous envelope. None of these methods are entirely satisfactory since they are all dependent upon the helix having the proper pitch when it is inserted within the envelope and maintaining the pitch while the helix is fixedly secured. Furthermore, in those instances where the helix is partly embedded in the vitreous envelope, dielectric loading is increased. None of the methods of the prior art permit adjustment of the pitch after the helix is seated.

It is a general object of the present invention to provide an improved assembly method for-travelling wave tube having a helical slow wave structure.

It is another object of the present invention to provide a method for mounting a helix on a supporting structure in such a manner that the helix has accurate and uniform pitch and it is securely retained by the supporting structure.

It is another object of the present invention to provide an improved method for placing a helix in an elongated cylindrical envelope.

It is still another object of the present invention to provide an improved method for placing a helix'in a travelling wave tube envelope so that the helix is fixedly seated within the envelope and has accurate spacing between adjacent convolutions. g

It is another object of the present invention to provide a method of mounting a helix on a supporting structure which comprises placing the helix in cooperation with the structure, setting the helix on the supporting structure and then positioning the helix to obtain a'uniform pitch.

It is another object of the present invention to provide a method of mounting a helix on a support which comprises. placing the helix in cooperation with the supporting structure, urging the helix against the support and maintaining pressure between the helix and support so that the spacing is retained by the restraining frictional forces created between the support and helix.

It is still another object of the present invention to provide a method of mounting a helix on a supporting structure which comprises placing the helix in cooperation with the supporting structure, causing the turns of the helix to move radially to press against the support, and

holding the helix whereby the pressure is maintained.

It is a further object of the present invention to provide a method of mounting a helix in a supporting struc- It'the pitch of the helix varies during operation, the oscillation frequency will be ture which comprises placing the helix in cooperating relationship with the structure, employing a guide mandrel to position the helix on the structure, causing the turns of the helix to move radially to press against the support, and

holding the helix whereby the pressure is maintained.

It is still a further object of the present invention to provide. a method of mounting a helix on a supporting structure which comprises placing the helix in cooperative relationship with the supporting structure, inserting co Ce Patented Dec. 15, 1964 rods;and- I FIGURE'23 isa sectional view showingthe helix,

axially of the helix a mandrel having a helically arranged groove adapted to receive and position the helix so that it conforms to the final arrangement, moving a probe progressively along said mandrel to progressively urge the helix turns radially against the supporting structure, and securing the helix whereby the radial force is maintained.

It is still a further object of the present invention to provide an improved apparatus for mounting a helix on a supporting structure which comprises a guide 'mandrel These and other objects of the invention will be more.

clearly understood from the following description taken in conjunction with theaccompanying drawing.

Referring to the drawing:

FIGURES lA-lH schematically illustrate one method for forming a travelling wave tube assembly in accordance with the present invention;

FIGURE 2is an enlarged sectional view taken along the line 2 2 of FIGURE 132; v v

FIGURE 3 is an enlarged view of a portion of the positioning mandrel; 7

FIGURE 4 schematically shows the engagement of the helix by the positioning mandrel;

FIGURE 5 is an enlarged sectional view taken along the line 5-5 of FIGURE lG'and shows the helix as set and inserted in the elongated glass envelope;

FIGURES 6A'6D show another method of forming a travelling wave tube assembly in accordance with the present invention;

FIGURE 7 is an enlarged sectional view taken along the line 7-701 FIGURE 6A;

FIGURE 8 is an enlarged sectional view taken along the line 88 of FIGURE 6D;

FIGURE 9 is a side elevational view, partly in section, of a mandrel; 7

FIGURE 10 is a sectional view taken along the line 1tllld of FIGURE 9;

FIGURE 11 is a side elevational view of a setting mandrel together with its setting member;

FIGURE 12 is a sectionalview taken along the line 12 412 of FIGURE 11; 7

FIGURE 13 is a side elevational view of another setting mandrel suitable for setting small helices in accordance with the invention;

4 a helix assembly in accordance with the present invention is schematically illustrated. In FIGURE 1A, there is shown a helix 1.1 which is formed by helically winding tape or ribbon material. The helixmay, for example, be formed by winding the tape or ribbon'material on a smooth mandrel. In the present embodiment of the invention the size of the mandrel is selected whereby when the material that is wound is released, it will attain a normal outside diameter which is slightly larger than the diameter of a circle'which is coincident with the exterior supporting means for the helix, as will be presently described. When internal supporting means are employed,

the helix will have an internal diameter slightly less than that of a circle coincident with the supporting surfaces.

The helix II is adapted to be inserted in a travelling wave tube envelope 12. The envelope 12 includes a relatively long cylindrical portion 13 which expands into a bell 14 at one end. 7 The bell serves to receive an electron gun (not shown) for projecting an electron stream axially of the helix and envelope.

A The envelope portion 13 may include inwardly extending longitudinal ridges 16 FIGURE 2 which may, for example, be formed by shrinking the cylindrical glass envelope on a suitable mandrel. The surfaces 17 are accurately formed so that they define points on a cylindrical surface. The diameter of the surface is less than the normal outer diameter of a helix supported thereby. v

diameter which is slightly greater than the diameter of the FIGURE 14 is a sectional view of the mandrel shown in FIGURE 13;

FIGURE 15 shows a perspective of still another type of setting mandrel;

FIGURE 16 is a sectional view taken along the line 7 by an internal cylindrical surface which includes the supporting surfaces 17. To insert the helix II in the envelope,'an inserting tool is provided. For example, the tool may comprise a rod or mandrel 21 which receives the helix and holds it at one end. The rod is of such diameter that when the helix is tightly wound'thereon, the assembly fits into the envelope. To place thehelix in the envelope, the rod is rotated in such a direction that the helix tends to wind tightly on the same, and simultaneously the assembly is moved axially into the envelope portion 13 (a screw action). Subsequently, one end of the helix is released from the mandrel allowing the helix to unwind and expand. The turns nearest each end will be in intimate contact with the glass surfaces 17 due to the radial spring forces of the helix turns. It is apparent that if each turn is allowedto unwind there will be suflicient pressure outwardly against surfaces 17 to cause the helix to be firmly held against axial movement. It is apparent that if sufficient pressure is present, the helix will be firmly held against axial movement by the friction between the ground glass surface 1'7 and the contacting portion of the convolutions of the helix; The friction forces are such that the helix is not displaced by movement of the tube, normal vibrations, or jarring motions.

A suitable rod or mandrel 21 for inserting (threading) the helix into the envelope is shown in FIGURE 1B1. It

. includes a mandrel or rod portion 21 which accommodates FIGURE 19 is a sectional view ofa mandrel suitable for mounting the helix on an internalsupport;

FIGURE 20 shows a setting mandrel in operation to set the helix on an internal support;

21-21 of FIGURE 20;

FIGURE 21 is a sectional viewtaken along the line FIGURE 22. is a perspective view showing'themethod s of the present invention for positioninga helix on spaced mandrel and spaced rods in a fixture.

a plunger 22. The end of the plunger includes an ear 23 whichis adapted to abut against the side '24 of the adjacent convolution of a helix placed on the mandrel. Onfthe opposite side of the convolution, there is a fixed ear-2f. Thus, by spring loading the plunger 22 so that it is urged to the left as viewed in FIGURE 151, the helix is pinched between the ears 23 and 26. As previously described, the outer diameterof the mandrel 21 is selected whereby when the helix is tight against the mandrel, there will be a clearance'between the outside of the helix and the interior supporting surfaces 17 of the envelope.

i To insert; the helix, theinserting mandrel is guided within the tubejFIGURE 182. and rotated in the same senseas the convolution of the helix (see FIGURE 10), wherebyit is in efiect threaded or screwed into the'envelope. As the mandrel is rotated, the convolutions which engage the supporting surfaces 17 are retarded by friction and there is a winding up of the helix onto the mandrel so that the convolutions attain a diameter less than the diameter of the cylindrical surface including the supporting surfaces 17. The helix may be advanced into the envelope until it is longitudinally spaced within the envelope. Subsequently, the plunger 22 is released to release the helix. The helix tends to spring outwardly providing clearance for withdrawal of the inserting tool and giving rise to pressure between the contacting portions of the convolutions and the surfaces 17. FIGURE 1]) schematically shows the helix in the envelope.

The next step is to insert a master mandrel which has formed thereon a helical groove 28 FIGURES 1E and 3, having standard accurate pitch, conforming to the desired pitch for the helix. The width of the groove is slightly greater than the width of the tape forming the helix. The grooves are inclined and lapped to the proper size and pitch. The master mandrel has a major diameter less than the support diameter and a minor diameter less than the final inside diameter of the helix. The mandrel may be provided with a handle portion (not shown) and the helical groove is preferably longer than the longitudinal extent of the helix.

The mandrel is then, in effect, threaded or screwed into the helix within the tube envelope, FIGURE 1F, and, as it is rotated and moved into the helix, it will move the individual convolutions by overcoming the frictional retaining force to adjust the pitch of the helix to conform to that of the mandrel groove. Since the grooves are wider than the tape the mandrel is urged in one direction whereby the same side of all of the grooves engages the same side of the helix. In this manner, the pitch between corresponding edges of the helix is accurately adjusted.

After the mandrel is completely threaded into the helix and the helix positioned, the assembly, including the mandrel, is vibrated at a relatively high frequency, for example, an ultrasonic frequency. The friction between the convolutions of the helix and the adjacent supporting surfaces becomes negligible. The helix will tend to regain its normal diameter and pitch. In effect, the helix unwinds until the pressure between convolutions and supporting surfaces is equalized along the length of the envelope. The two ends of the helix are welded to the leadhis 30 and 31, FIGURE 1D. The same, FIGURE 4,

or another master mandrel may then be threaded into the helix one turn at a time, and moved to and fro. This overcomes the frictional retaining force and individually and sequentially imparts. the proper pitch to the helix by assuring that the same edge of the helix abuts the same edge of the groove. so that the frictional forces between the helix and adjacent supporting surfaces can be overcome for positioning. The mandrel is then removed, FIGURE 1H. This final positioning step is only necessary where it is desired to have extreme accuracy; It is apparent that even without this step, the accuracy of positioning is relatively high since the convolutions are positioned by the standard mandrel during the vibratory setting operation.

The pitch of'the helix will be substantially uniform (as uniform as the mandrel plus'tape width variations) and will be maintained by friction forces'created by the pressure exerted by the helix against the supporting surfaces due to the spring action of the helix; see FIGURES 1H and 5'.

In summary the steps are (1) inserting the helix within the tube envelope by threading; (2) positioning the helix by inserting a master mandrel; (3) setting the. helix within the envelope by allowing it to unwind, thus equalizing the pressure and retaining the helix in accurate. position by friction; (4) if desired, accurately adjusting the pitch by employing the same or another mandrel for a final positioning.

In the prior art methods, the pitch of the helix is deter- This is done a turn or two at a time mined by-the accuracy with which the helix is wound. In the present invention, the pitch is determined by the master mandrel.

It is apparent, of course, that a similar method may be followed in positioning a helix in an envelope which does not include a plurality of supporting ridges. It is preferable to use an envelope which has ridges since the use of ridges reduces the amount of dielectric in contact with the helix to lower the dielectric loading factor and raise the helix impedance. This, in turn, allows better coupling to the beam, widens the bandwidth of the tube, and raises its efiiciency.

The vibration referred to above may comprise immersing the assembly in an ultrasonic bath. The fluid in the acts as a lubricant between the helix and supporting surfaces and allows the use of lower power in the vibrator.

An alternative method for setting the helix in the envelope is illustrated in FIGURES 6AD. Rather than placing a solid setting mandrel in the sub-assembly as shown in FIGURE 1F, a split mandrel 41, FIGURES 6A and 9-14, is wound into the helix to adjust its position. The split mandrel carries a finned member 48 FIGURES 7 and 12, which progressively urges the convolutions outwardly as it is moved longitudinally to force the helical turns mechanically outwardly and firmly seats the same against the supporting surfaces. The action as the finned member engages the convolution is to spread theconvolution outwardly, unwinding the helix from the'end which .is not engaged. As will become apparent, the

method set forth in this embodiment can be carried out for a helix supported internally the internal diameter is preferably less than the support, but it may be slightly greater.

Referring to FiGURES 9-12, a suitable split mandrel is illustrated. Thus, the mandrel 41 is machined and lapped to form a helical groove 42 which is adapted to receive and position the helix within the tube envelope. The groove has a pitch which is the desired pitch of the helix, a major diameter less than the supporting structure, and a minor diameter less than the final internal helix diameter. The mandrel is slotted as indicated at 43, FIG- URES 9 and 10, and includes a cylindrical opening 44. The opening 44 is adapted to receive a cylindrical rod (not shown) which serves to provide rigidity to the structure whereby the individual segments of the mandrel are retained in spaced relationship.

Referring to FIGURES 11 and 12, a second rod 47 is provided which includes finsxor presser portions 48 extending outwardly from the same. extend outwardly beyond the surface of the mandrel and a cylinder which includes the surface of these. fins is slightly greater in diameter than the inside surface of the helix when the helix is in contact with the supporting surfaces 17 of the envelope, FIGURE 7.

Toform a helix assembly, the helix is placedincooporation with the supporting structure. If the helix is such that its diameter normally places it in. contact with the' 49. 'The rod 4-6 is then inserted into the cylindrical opening 44. The mandrel is then threaded into the. helix and until it is completely threaded to adjust the pitch of the helix... At thistirne, the rod 47 is urged progressively along themandrel whereby the finned structure 48 sequentially engages the turns of the helical conductor as illustrated in FIGURES 6A and ,7. The convolutions are urged outwardly whereby the helix is expanded'and bears against the supporting surfaces 17 As'the' fin structure 7 The grooved fins 48.

The rod 47 and fin structure 43 are inserted- 9. A method as in claim 6 wherein said helix is placed in cooperative relationship with an internal supporting surface and progressively contracted radially inwardly.

10. A method as in claim 9 wherein the normal diameter of said helix is smaller than the diameter of the supporting structure.

11. A method of mounting a helix to a supporting structure which comprises placing the helix in cooperative relationship with a supporting structure by engaging one end of the helix and simultaneously rotating and advancing the helix into the supporting structure, inserting a mandrel in cooperative relationship with the helix, said mandrel having a helical groove of the desired final configuration adapted to receive the helix and position the same, and subsequently causing the helix to move radially against the supporting structure to be firmly engaged thereby.

12. A method of mounting a helix to a supporting structure which comprises placing the helix in cooperative relationship with the supporting structure, inserting a mandrel in cooperation with the helix, said mandrel having a helical groove of the desired final configuration adapted to receive the helix and position the same, subsequently causing the helix to move radially against the supporting structure to be firmly engaged thereby, securing the ends of said helix, and finally adjusting the pitch of the helix by inserting another mandrel having a helical groove for receiving and positioning the helix.

13. The method of mounting a helix in an external supporting structure which comprises placing a helix whichhas an outside normal diameter slightly greater than the inside diameter of the external supporting structure within the structure, inserting a mandrel in cooperative relationship with the helix, said mandrel having a helical groove of desired final configuration adapted to receive and position the helix, vibrating the assembly to cause the helix to move radially outwardly against the supporting structure to be firmly engaged thereby, securing the ends of said helix, and inserting another mandrel to finally position the helix within the structure.

14. The method of mounting a helix to a supporting structure which comprises placing the helix in cooperative relationship with said supporting structure, securing one end of said helix, inserting a mandrel in cooperation with said helix, said mandrel initially engaging the helix at the secured end, and having ahelical groove of desired final configuration for receiving and positioning the helix, progressively applying a radial force to said helix to cause the same to move radially against the supporting structure to be firmly engaged thereby, securing the other end upon completion of the radial expansion of the helix whereby the helix is firmly maintained by the supporting structure, and inserting another mandrel having a helical groove for receiving and positioning the helix to finally position the helix Within the structure.

15. The method of mounting a helix in a travelling wave tube envelope which comprises placing the helix coaxially within said envelope and in cooperative relationship therewith, inserting a mandrel having a groove of desired final configuration in cooperation with said helix to receive and position the same in accordance with the groove, and subsequently causing the helix to move radially outwardly against the supporting structure to be firmly engaged thereby for accurate retention by the forces between the tube envelope and helix.

16. The method of mounting a helix in a tubular envelope which comprises placing a helix which has a normal diameter slightly greater than the inside diameter of the tubular structure within the same, inserting a mandrel in cooperative relationship with the helix, said mandrel having a helical groove of desired final configuration adapted to receive and position the helix, and subsequently vibrating the assembly to cause the helix to move radially outwardly against the tubular envelope to be firmly engaged and retained thereby.

17. The method of mounting a helix in a tubular envelope which comprises placing the helix in cooperative relationship within the envelope, securing one end of the helix, inserting a mandrel in cooperation with said helix, said mandrel initially engaging the helix at the secured end, said mandrel having a helical groove of desired final configuration for receiving and positioning the helix, progressively applying a radial force to said helix to cause the same to move radially outwardly against the envelope to be firmly engaged thereby, and securing the other end upon completion of the radial expansion of the helix whereby the helix is firmly maintained by the envelope.

18. Apparatus for mounting a helix in a supporting structure comprising a mandrel having a helicalperiphery accurately conforming to the desired final arrangement of the helix in the supporting structure, said mandrel including at least one longitudinal slot, and a probe longitudinally movable in said slot and projecting beyond the inner surface of helix to be tightened for applying a radial force progressively along the mandrel to a helix supported thereon.

19. Apparatus for mounting a helix on a supporting structure comprising a segmented mandrel having a helical periphery accurately conforming to the desired final arrangement of the helix in the barrel and having an outer diameter slightly greater than the internal diameter of the supporting structure, said mandrel having at least one longitudinally extending opening therein, and a probe movable longitudinally of said mandrel and having at least one projection extending through said opening to outwardly direct force progressively along the mandrel.

20. Apparatus for mounting a helical conductor on a supporting structure comprising a hollow mandrel, said hollow mandrel having an internal helical groove conforming to a desired configuration, and at least one longitudinal slot adapted to receive a projecting fin adapted to extend beyond the surface of said mandrel and adapted to progressively urge a helix associated therewith inwardly.

21. A mandrel for mounting a helix in a supporting structure comprising an elongated rod having a helical peripheral groove accuratelly conforming to the desired final arrangement of a helix in a supporting structure, a plurality of longitudinal slots formed on said rod, a

. finned structure including a fin extending through each of said slots and projecting beyond the outer surface of said mandrel to thereby provide an outward force to a helix mounted thereon, and means for longitudinally moving said fin whereby they progressively engage a helix mounted in cooperation therewith to seat the same against an external supporting structure.

22. A method of mounting a helix to a supporting structure in a helix travelling wave tube which comprises the steps of placing the helix in cooperative relationship with the supporting structure to be supported and held thereby, inserting a mandrel having a helical groove of the desired final configuration and pitch in cooperative relationship with said helix to progressively receive the helix in the groove and position the same in accordance therewith, and subsequently removing the said mandrel from said helix and supporting structure.

References Cited in the file of this patent UNITED STATES PATENTS 2,210,061 Caminez Aug. 6, 1940 2,708,306 Lampton May 17, 1955 2,822,501 Poulter Feb. 4, 1958 2,845,690 Harrison Aug. 5, 1958 2,850,666 Brewer Sept. 2, 1958 2,851,630 Birdsall Sept. 9, 1958 2,869,217 Saunders Jan. 20, 1959 2,943,228 Kleinman June 28, 196.0 

4. THE METHOD OF MOUNTING A HELIX IN AN EXTERNAL SUPPORTING STRUCTURE WHICH COMPRISES PLACING A HELIX WHICH HAS AN OUTSIDE NORMAL DIAMETER SLIGHTLY GREATER THAN THE INSIDE DIAMETER OF THE EXTERNAL SUPPORTING STRUCTURE WITHIN THE STRUCTURE, INSERTING A MANDREL IN COOPERATIVE RELATIONSHIP WITH THE HELIX, SAID MANDREL HAVING A HELICAL GROOVE OF THE DESIRED FINAL CONFIGURATION ADAPTED TO RECEIVE THE HELIX IN THE GROOVE AND POSITION THE HELIX, AND SUBSEQUENTLY VIBRATING THE ASSEMBLY TO CAUSE THE HELIX TO MOVE RADIALLY OUTWARDLY AGAINST THE SUPPORTING STRUCTURE TO BE FIRMLY ENGAGED THEREBY. 